"Christma-eve" before 2000 throws NPE

[simonwibberley]

Thanks for a brilliant library, works almost perfectly :) thanks!

However, "Christmas-eve" fails with reference years before 2000 (2000+ works).

Parser parser = new Parser();
        List<DateGroup> dateGroups = parser.parse("Christmas-eve", new Date(LocalDateTime.of(1999, 3, 1, 0, 0, 0).toInstant(ZoneOffset.UTC).toEpochMilli()));
        if(dateGroups.size() == 1) {
            List<Date> parsed = dateGroups.get(0).getDates();
            if(parsed.size() == 1) {
                System.out.println(parsed.get(0));
            }
        }

Stack trace:

Exception in thread "main" java.lang.NullPointerException
	at java.util.Date.getMillisOf(Date.java:958)
	at java.util.Date.after(Date.java:931)
	at com.joestelmach.natty.WalkerState.seekToIcsEvent(WalkerState.java:571)
	at com.joestelmach.natty.WalkerState.seekToHoliday(WalkerState.java:393)
	at com.joestelmach.natty.generated.DateWalker.seek(DateWalker.java:1376)
	at com.joestelmach.natty.generated.DateWalker.relative_date(DateWalker.java:713)
	at com.joestelmach.natty.generated.DateWalker.date(DateWalker.java:659)
	at com.joestelmach.natty.generated.DateWalker.date_time(DateWalker.java:585)
	at com.joestelmach.natty.generated.DateWalker.date_time_alternative(DateWalker.java:532)
	at com.joestelmach.natty.generated.DateWalker.parse(DateWalker.java:418)
	at com.joestelmach.natty.Parser.singleParse(Parser.java:220)
	at com.joestelmach.natty.Parser.parse(Parser.java:112)
	at uk.ac.susx.tag.method51.text.pipeline.DateParser.main(DateParser.java:117)